Well it is going to depend on ... 1. If the phone is running Android "Marshmallow" or can be upgraded to it, and 2. If the manufacturer has restricted this option on this phone. They do that sometimes.
Here are the instructions on how to do it.
How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device).
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and...